If you want to filter the reference spectrum first as i suggested in the tutorial in the other thread “ALHENA WITH A STARANALYSER”
https://britastro.org/node/11586
https://britastro.org/sites/default/files/attachments/Correcting%20for%20instrument%20response.pdf
then the spline filter function in Visual Spec can be found at “Operations”, “Spline Filter”
it is also a good idea to roughly rescale the spectra first before calculating the instrument response so you dont have to deal with very large numbers. You can do this by selecting a region of the spectrum using the cursor (mouse left click and drag), then right click and select “normalize”. The result is the spectrum is averaged to 1 over the selected region
Cheers
Robin